The present invention relates to a heat-resistant organopolysiloxane composition or, more particularly, to an organopolysiloxane fluid having improved heat-resistance.
As is known, one of the methods for the improvement of the thermal stability of an organopolysiloxane is the admixture of the organopolysiloxane with an inorganic compound of a metal such as iron, zirconium, cerium, manganese and the like. It is also known that cerium compounds are the most preferable among the inorganic compounds of the above mentioned metal compounds in respect of the excellent thermal stability of the organopolysiloxane admixed therewith and no or little coloration of the organopolysiloxane thereby. Suitable forms of the cerium compounds include the oxide, hydroxide and salts with aromatic carboxylic acids (see, for example, Japanese Patent Publications Nos. 43-16304 and 36-6189). A problem in the admixture of these cerium compounds with organopolysiloxanes in general is the low solubility of the cerium compound in the siloxane so that the cerium compound can hardly be introduced into the organopolysiloxane with sufficient uniformity and the improvement of the thermal stability obtained thereby is not sufficiently high as a consequence.
Accordingly, various attempts and proposals have been made in connection with the method for uniformly introducing a cerium compound into an organopolysiloxane though with their own respective disadvantages and problems. For example, a chelate compound of cerium is reacted with an organopolysiloxane having hydrogen atoms directly bonded to the silicon atoms and the reaction product is added to an organopolysiloxane (see U.S. Pat. No. 3,008,901). A problem in this method is the difficulty to ensure complete absence of the hydrogen atoms directly bonded to the silicon atoms which are detrimental to the thermal stability of the organopolysiloxane. In another method, a silicone fluid containing acetylacetonato complex of cerium dissolved therein is added dropwise into another silicone fluid heated at 200.degree. to 300.degree. C. while air is blown thereinto (see Japanese Patent Kokai No. 53-65400). This method is disadvantageous due to the expensiveness of the cerium complex in addition to the limited solubility thereof in silicone fluids which consequently necessitates an increased amount of addition into the silicone fluid. Further, the dropwise addition of the silicone fluid containing the cerium complex must be carrried out with utmost care in order to avoid precipitation of the cerium complex taking a long time so that the process is complicated. Further, a method is proposed in which a cerium salt of a carboxylic acid is reacted with an alkali metal silanolate and the reaction product is added to an organopolysiloxane together with an alkoxide or carboxylate of zirconium, titanium, iron and the like metal (see Japanese Patent Publication No. 53-12541). A problem in this method is that, while the reaction product of the cerium salt and the alkali silanolate must be neutralized before introduction into the organopolysiloxane, difficulties are encountered in achieving an exact neutrality always leaving a trace amount of an acidic or alkaline ionic impurity which is very detrimental to the thermal stability of organopolysiloxanes so that the desired imporvement in the thermal stability of an organopolysiloxane can not always be obtained.